Core material for vacuum heat insulation material, and vacuum heat insulation material

ABSTRACT

Description is made of an open cell polystyrene resin foam having a content of organic volatile matters of less than 100 ppm. The open cell foam is suitable as a core material for a sound insulating material or a vacuum heat insulating material. 
     The open cell foam can be produced by melt-kneading a polystyrene resin having a content of organic volatile matters of 100 ppm or higher together with water in an extruder, sucking out the organic volatile matters as an azeotrope with water through a vent, injecting a blowing agent and kneading the injected mixture, and extruding the mixture into a lower pressure area to cause the resin to foam.

TECHNICAL FIELD

The present invention relates to a foam having an open cell structure(open cell foam) comprising a polystyrene resin and a method forproducing the foam. The present invention also relates to a corematerial for a vacuum heat insulating material and to a vacuum heatinsulating material.

BACKGROUND ARTS

A vacuum heat insulating material produced by packing a core materialhaving shape retention properties with a gas barrier film and evacuatingthe inside of the film exhibits good heat insulating properties byevacuating gas remaining in the core material. Thus, a material havingcompressive strength to withstand the evacuation and being capable offorming a vacuum insulation space is used for the core material.

Conventionally known examples of the core material for a vacuum heatinsulating material include (1) a material obtained by solidifyinginorganic fine particles such as silica powder or perlite powder; (2)open cell polyurethane foam; and (3) a material obtained by solidifyingglass fibers. The material (1) is made of super fine particles and thuscauses a problem of dust particles in the production and disposalprocesses. The material (2) is easy to handle but difficult to recycle.Also, the material (2) needs two to three hours' predrying at 120° C. asa pretreatment prior to vacuum packing because the resin contains aminor amount of volatile matters such as water and carbon dioxide gas.The material (3) has a problem in handling caused by scattering of glassfibers as in the case with inorganic powders.

Recently, a polystyrene open cell foam for a vacuum heat insulatingmaterial has been disclosed in PC(WO)H11-504362. Such a polystyreneresin foam, which is light in weight and easy to handle, is excellent asa core material for a vacuum heat insulating material. However, a vacuumheat insulating material obtained using the foam for a core has a greatchange in its heat insulating properties with time and does not havesatisfactory quality.

An object of the present invention is to provide an open cellpolystyrene resin foam suitable as a core material for a vacuum heatinsulating material and a method for producing the foam. Another objectof the present invention is to provide a core material for a vacuum heatinsulating material comprising the foam and a vacuum heat insulatingmaterial produced using the core material.

DISCLOSURE OF INVENTION

As a zealous studies to accomplish the above objects, the presentinventors found that the reason why a vacuum heat insulating materialproduced by packing an open cell foam of a polystyrene resin with a gasbarrier film and evacuating the inside of the film has a great change inits heat insulating properties with time is that the foam containsorganic volatile matters including styrene derivatives in addition toremaining blowing agent and this causes a great change in the degree ofvacuum in the vacuum package with time, and has accomplished the presentinvention.

According to the present invention, there are provided an open cellfoam, a method for producing the open cell foam, a core material for avacuum heat insulating material comprising the foam, and a vacuum heatinsulating material produced using the core material as follows:

-   (1) An open cell foam characterized in that said open cell foam    comprises a polystyrene resin foam having an open cell content of at    least 90%, a density of 0.03–0.4 g/cm³ and a cell diameter in the    direction of the thickness of said foam of less than 400 μm, and in    that said polystyrene resin foam has a content of organic volatile    matters (organic matters having a boiling point of not higher than    160° C. under ambient pressure) of less than 100 ppm.-   (2) An open cell foam as recited in (1), characterized in that said    polystyrene resin foam has a content of organic volatile matters of    50 ppm or less.-   (3) An open cell foam as recited in (1), characterized in that said    polystyrene resin foam has a content of organic volatile matters of    less than 20 ppm.-   (4) An open cell foam as recited in any one of (1) to (3),    characterized in that said polystyrene resin foam has a cell    diameter in the direction of the thickness of said foam of 80 to 200    μm.-   (5) An open cell foam as recited in any one of (1) to (4),    characterized in that said polystyrene resin foam has a thickness of    5.0 to 50.0 mm.-   (6) A method for producing an open cell foam according to (1) having    a content of organic volatile matters of less than 100 ppm,    characterized in that said method comprises melt-kneading a    polystyrene resin having a content of organic volatile matters of    less than 100 ppm together with a blowing agent in an extruder, and    then extruding the kneaded mixture into a lower pressure area to    cause said polystyrene resin to foam.-   (7) A method for producing an open cell foam according to (1),    characterized by comprising melt-kneading a polystyrene resin    together with water in an extruder, sucking out volatile matters    through a vent as an azeotrope with water, injecting a blowing agent    and kneading said polystyrene resin therewith, and extruding the    kneaded mixture into a lower pressure area to cause said polystyrene    resin to foam.-   (8) A core material for a heat insulating material comprising an    open cell foam according to any one of (1) to (5) and having a    density of 0.03 to 0.2 g/cm³ and a 5% compression strength at 23° C.    of at least 0.05 MPa.-   (9) A method for producing an open cell foam according to claim 6 or    7, characterized in that said blowing agent comprises, as a main    component, at least one blowing agent selected from propane, methyl    chloride, dimethyl ether, methyl ethyl ether and diethyl ether.-   (10) A vacuum heat insulating material produced by covering a core    material according to (8) with a gas barrier film, and evacuating    the inside of said film.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of polystyrene resins for use in the present invention includepolystyrene resins and polystyrene resin compositions having a styrenecomponent content of at least 50% by weight (preferably at least 60% byweight, more preferably at least 80% by weight) such as styrenehomopolymer resins, styrene copolymer resins produced from styrene andanother monomer, mixtures of a styrene homopolymer resin and/or astyrene copolymer resin with a styrene-conjugated diene block copolymeror its hydrogenation product, rubber-modified styrene resins (impactresistant polystyrene) obtained by polymerization of a styrene monomerin the presence of a rubber-like polymer, and mixtures of the abovestyrene resins with another resin and/or another rubber like(co)polymer.

Specific examples of the styrene copolymer resins includestyrene-acrylonitrile copolymer, styrene-butadiene-acrylonitrileterpolymer, styrene-acrylic acid copolymer, styrene-methacrylic acidcopolymer, styrene-methyl methacrylate copolymer, styrene-ethylmethacrylate copolymer, styrene-methyl acrylate copolymer, styrene-ethylacrylate copolymer, styrene-maleic anhydride copolymer, andpolystyrene-polyphenylene ether copolymer.

Above all, the base resin for the open cell foam is preferably apolystyrene resin having a Vicat softening point of 110° C. or higher sothat the core material of the vacuum heat insulating material can havesufficient heat resistance not to be deformed by heat. The upper limitof the Vicat softening point is about 140° C. for reasons of preventingthe extrusion foaming temperature of the resin for obtaining an opencell foam from rising to such an extent that the decomposition reactionof the styrene component occurs. Especially preferred is the use ofstyrene-acrylic acid copolymer, styrene-methacrylic acid copolymer,styrene-methyl methacrylate copolymer, styrene-ethyl methacrylatecopolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylatecopolymer, or styrene-maleic anhydride copolymer because of their highheat resistance. A vacuum heat insulating material having a core of anopen cell foam comprising the above resin exhibits an especially lowthermal conductivity. The reason for that has not understood yet, but itis considered to be derived from the thermal conductivity of the resins.

The Vicat softening point is a value obtained according to JIS K7206(1999, test load: Method B, by a fluid heating method under a heatingrate of 50° C./hr). The samples are not subjected to annealing.

Examples of the above-described “another resin” include polyethyleneresins such as low-density polyethylene, high-density polyethylene, andstraight chain low-density polyethylene; polypropylene resins such aspolypropylene, propylene-ethylene copolymer, and propylene-1 butanecopolymer; polyolefin resins such as cyclic polyolefin; polycarbonateresins; polyester resins such as polyester of polyethyleneterephthalate, polybutylene terephthalate, or polyethylene naphthalate;and polyphenylene ether resins. Examples of the above-described “anotherrubber like (co)polymer” include polyolefin thermoplastic elastomer, andpolyester thermoplastic elastomer.

Various additives, such as cell regulating agent (nucleating agent),antioxidant, thermal stabilizer, antistatic agent, conductivityimparting agent, weathering agent, ultraviolet absorber, colorant, flameretardant, and inorganic filler, may added to the polystyrene resin tosuch an extent that they do not interfere with the objects of thepresent invention.

Illustrative of blowing agents suitable to obtain a foam with a highopen cell content of the present invention are chlorinated hydrocarbonssuch as methyl chloride, methylene chloride, and ethyl chloride;chlorofluorocarbons such as 1,1,1-trichloroethane,1,1-dichloro-1-fluoroethane (HCFC-141b),1,1-dichloro-2,2,2,-trifluoroethane (HCFC-123), chlorodifluoromethane(HCFC-22), and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124); aliphatichydrocarbons such as methane, ethane, propane, n-butane, isobutane,n-pentane, isopentane, and neopentane; fluorocarbons such as1,1-difluoroethane (HFC-152a), 1,1,1-trifluoroethane (HFC-143a),1,1,1,2-tetrafluoroethane (HFC-134a), and difluoromethane (HFC-32);alcohols, carbon dioxide (CO₂), water, and nitrogen. The blowing agentsmay be used alone or in combination. As a blowing agent to obtain a corematerial for a vacuum heat insulating material, one which quicklydiffuses into the air from the inside of the foam is preferred.Illustrative of such blowing agents are chlorinated hydrocarbons such asmethyl chloride, ethyl chloride, and ethylene chloride; alcohols,propane, dimethyl ether, methyl ethyl ether, diethyl ether, water,nitrogen, and carbon dioxide. The blowing agents may be used alone or incombination.

Above all, the blowing agent is preferably composed mainly of at leastone blowing agent selected from propane, methyl chloride, ethyl chloridedimethyl ether, methyl ethyl ether and diethyl ether because an opencell foam having an high open cell content can be easily obtained with ahigh productive efficiency. Especially preferred is the use of a blowingagent mainly composed of at least one blowing agent selected from methylchloride, propane, and dimethyl ether because the foaming temperaturerange of these blowing agents is so wide that an open cell foam can beeasily obtained. The blowing agent may contain 5–50 mol % of water,nitrogen or carbon dioxide to ensure safety against ignition and safetyof humans and to obtain open cells more easily. To “be composed of atleast one blowing agent” herein means to contain the blowing agent(s) atleast 50 mol % when the total amount of the blowing agent is taken as100 mol %.

In the present invention, the amount of the blowing agent, which isdetermined on the density of the intended foam, is generally 0.05–2.5mol per 1 kg of the material resin to obtain a foam with a density of0.03–0.4 g/cm³.

The open cell foam of the present invention obtained using a polystyreneresin has a density of 0.03–0.4 g/cm³ An open cell foam having a densityof over 0.4 g/cm³ exhibits so poor heat insulating properties that ahigh-performance heat insulating material cannot be obtained. An opencell foam having a density of lower than 0.03 g/cm³ tends to haveinsufficient compressive strength. For example, when such an open cellfoam is vacuum packed, the cells cannot withstand the difference betweenthe vacuum pressure in the wrapping material and the ambient pressureand are deformed. Thus, it is difficult to obtain a good vacuum heatinsulating material.

The open cell foam of the present invention obtained using a polystyreneresin has a cell diameter T in the direction of the thickness of thefoam of less than 400 μm, preferably not greater than 300 μm. Apolystyrene resin open cell foam having a cell diameter T in thedirection of the thickness of the foam of not smaller than 400 μmexhibits poor heat insulating properties. The lower limit of the celldiameter T is preferably about 50 μm.

The control of the cell diameter of the open cell foam of the presentinvention, the method for which is dependent on the physical propertiesof the polystyrene resin and the density of the foam, is made bychanging the content of carbon dioxide or nitrogen in an organic blowingagent such as a chlorofluorocarbon, aliphatic hydrocarbon orfluorocarbon or adding a cell regulating agent, such as talc, in anamount of 1–30 parts by weight per 100 parts of the polystyrene resin.

The cell diameter herein is an average cell diameter obtained by thefollowing method. Using a microscope or the like, an enlarged image of across-section of the foam in the extrusion direction or the widthdirection is projected on a screen or monitor. On the projected image, astraight line is drawn in the measuring direction (in the thicknessdirection when the cell diameter in the thickness direction is measured)and the number of cells through which the line passes is counted. Then,the average cell diameter is obtained by dividing the length of the line(which is not the length on the enlarged projected image but the reallength calculated taking the magnification into account) by the numberof the counted cells.

The polystyrene resin open cell foam of the present invention has anopen cell content of at least 90%, preferably at least 95%. The upperlimit of the open cell content is 100%. An open cell foam having an opencell content of 90% or higher exhibits excellent sound insulatingproperties and vacuum heat resistance properties. An open cell foamhaving an open cell content of lower than 90% cannot have a good heatinsulating effect since the closed cell in the foam cannot suckremaining foaming gas when the foam is vacuum packed to obtain a vacuumheat insulating material.

The open cell content of the polystyrene foam is obtained according toASTM D-2856-70 (Procedure C) as follows. The true volume Vx (cm³) of asample of the foam is measured with an air comparison pycnometer, andthe closed cell content is calculated by the formula (1). The apparentvolume Va (cm³) of the sample is the volume calculated from the outerdimension thereof. The true volume Vx (cm³) of the sample is the sum ofa volume of the resin constituting the foam and the total volume of allthe closed cells in the sample. Thus, the open cell content is obtainedby the following formula (1).Open cell content (%)=(Va−Vx)×100/(Va−W/ρ)  (1)wherein W represents the weight (g) of the sample, and ρ represents thedensity (g/cm³) of the base resin of the foam.

In the measurement of open cell content, a rectangular solid piece witha size of 25 mm long×25 mm wide×40 mm thick cut off from the foam isused as a sample. When a sample having a thickness of 40 mm cannot becut off from a foam, two or more samples may be used together. In thiscase, the samples are as close to the same size as possible, and thesmaller the number of the samples is, the better. Also, the total volumeof the samples calculated from the outer dimensions thereof should be 25cm³. The open cell content of the present invention is an arithmeticaverage of measurements of ten different samples randomly selected.

When the open cell foam of the present invention is used as a corematerial for a heat insulating material, it preferably has a density of0.03–0.2 g/cm³, more preferably 0.03–0.1 g/cm³ for reasons of preventionof deformation of the cells in vacuum packing and good thermalconductivity. The open cell foam of the present invention preferably hasa cell diameter T in the direction of the thickness of the foam of80–300 μm, more preferably 80–200 μm, most preferably 100–200 μm. Suchan open cell foam is suitable as a core material for a vacuum heatinsulating material.

The foam of the present invention has a content of organic volatilematters of less than 100 ppm (by weight), preferably 50 ppm or less,more preferably less than 20 ppm. Thus, the foam of the presentinvention is excellent in vacuum retention properties after vacuumpacking. The pressure within the package of a vacuum heat insulatingmaterial must be kept at 1–250 Pa in absolute pressure to maintain thethermal conductivity thereof. A polystyrene resin foam is thought tocontain organic volatile matters derived from styrene including styrenederivatives in addition to remaining blowing agent. Assuming that 150ppm of organic volatile matters come off from the foam, the vacuumdegree is reduced (the pressure in the package is increased) by 240 Pain terms of styrene monomers in theory. Namely, an open cell foamobtained using a polystyrene resin and having an organic volatilematters content of greater than 100 ppm is not suitable as a corematerial for a vacuum heat insulating material since the organicvolatile matters lower the vacuum degree after vacuum packing. The lowerlimit of the content of organic volatile matters in the foam is 0 ppm.

The organic volatile matters are organic matters having a boiling pointof not higher than 160° C. under ambient pressure. Illustrative of suchorganic matters are chlorinated hydrocarbons such as methyl chloride andethyl chloride; chlorofluorocarbons; fluorocarbons; aliphatichydrocarbons such as methane, butane, and propane; alcohols; ethers suchas dimethyl ether, methyl ethyl ether, and diethyl ether, which areincluded in the blowing agents; aromatic hydrocarbons such as xylene;and aromatic hydrocarbons derived from polystyrene such as toluene,ethyl benzene, propylbenzene, and styrene monomers.

The determinate quantity of the organic volatile matters is measured bygas chromatography as follows. 1 Gram of a sample cut off from a foam isput in a sample bottle containing a solvent such as dimethylformamideand an internal standard substance such as cyclopentanol to dissolve thesample in the solvent. The solution in the sample bottle is injectedinto a chromatograph to obtain a chromatogram.

More specifically, the determinate quantity of organic volatile mattersderived from polystyrene such as toluene, ethyl benzene, propylbenzene,and styrene monomers is measured by the following method. 1 Gram (whichcorresponds to Ws in the formula (2)) of a sample is cut off from apolystyrene resin or a foam and dissolved in 20 ml of dimethylformamidecontaining 0.001 g of cyclopentanol (5 g (which corresponds to Wi in theformula (2)) of cyclopentanol is dissolved in 10000 mL ofdimethylformamide to obtain an internal standard substance having acyclopentanol concentration of 5 g/10000 mL). With a microsyringe, 1 μLof the solution is injected into a gas chromatograph to obtain achromatogram. The conditions of gas chromatography are as follows.

The peak areas of each of the organic volatile matters and the internalstandard are obtained from the thus obtained gas chromatogram, and theconcentration of the organic volatile matter is obtained by the formula(2).Organic volatile matter concentration(ppm)=[(Wi/10000)×2]×[An/Ai]×Fn÷Ws×1000000  (2)wherein

-   Wi: weight (g) of cyclopentanol at the time when the internal    standard solution was prepared.-   Ws: weight (g) of the sample dissolved in DMF (dimethylformamide).-   An: peak area of each of the organic volatile matters at the time    when the gas chromatography was performed.-   Ai: peak area of the internal standard obtained at the time when the    gas chromatography was performed.-   Fn: correction coefficient for each of the organic volatile matters.

The correction coefficient for each organic volatile matter is obtainedas follows. Six working solutions containing an organic volatile matterand an internal standard (cyclopentanol) at different mixing ratios areprepared and injected into a gas chromatograph to obtain gaschromatograms to obtain a working curve for use in gas chromatography.The measurements of the organic volatile matter based on the gaschromatograms are plotted on a graph where the horizontal axisrepresents the weight ratio of the volatile matter content to theinternal standard in the working solutions [weight of volatilematter/weight of internal standard] and the vertical axis represents thepeak area ratio of the volatile matter to the internal standard [peakarea of volatile matter/peak area of internal standard] obtained by thegas chromatography. An example of the graph is shown in FIG. 1. Thegraph is approximated to a linear function by the least squares methodand the gradient of the graph is obtained. The gradient is used as thecorrection coefficient Fn for the volatile matter.

In the above formula, (Wi/10000) (g/mL) in [(Wi/10000)×2] represents theconcentration of the internal standard solution and 2 in [(Wi/10000)×2]means that 2 mL of the internal standard solution having theconcentration is used.

In the case of volatile matters which do not dissolve in a solvent suchas dimethylformamide, gas in the gas phase in the sample bottle can beused as the sample to be injected into the gas chromatograph.

The conditions of gas chromatography are as follows.

-   Device: Gas Chromatograph GC-6AM, manufactured by Shimadzu    Corporation.-   Column: glass column 3 mm in inside diameter and 5000 mm in length.-   Column filler: [liquid phase] FFAP (free fatty acid), [liquid phase    impregnation rate] 10% by weight, [support] diatomaceous earth for    gas chromatography, Chromosorb W, [support particle size] 60/80    mesh, [support treatment method] AW-DMCS (washing with water,    sintering, acid treatment and silane treatment), [amount] 90 mL.-   Inlet temperature: 250° C.-   Column temperature: 120° C.-   Detector temperature: 250° C.-   Carrier gas: N₂, flow rate 40 mL/min.-   Detector: FID (flame ionization detector).-   Determination: internal standard method.-   Detection limit: 20 ppm.

In the chromatogram obtained in the measurement of the amount of theorganic volatile matter, cyclopentanol as the internal standardsubstance and dimethylformamide as a solvent are also included. In thepresent invention, however, the contents of the internal standardsubstance and the solvent should be considered as 0 ppm.

Although description has been made of a specific organic volatilematter, other organic volatile mattes can be detected by changingconditions such an solvent, internal standard, inlet temperature, columntemperature, detector temperature, and column depending upon the organicvolatile mattes.

BRIEF DESCRIPTION OF DRAWINGS

The drawing is one example of a working curve used in measurement theamount of an organic volatile matter contained in a foam, wherein

-   A: weight ratio of an organic volatile matter (toluene) to internal    standard (cyclopentanol).-   B: peak area ratio of the organic volatile matter (toluene) to the    internal standard (cyclopentanol) in a gas chromatogram.

As described before, the foam of the present invention must have acontent of organic volatile matters of less than 100 ppm, preferably 50ppm or lower, more preferably less than 20 ppm, which cannot be detectedat a detection limit of 20 ppm (as mentioned before, the internalstandard substance and the solvent used in the measurement, which aredetected, are excluded). Most preferably, the foam has a content oforganic volatile matters of 0.

The styrene resin open cell foam of the present invention can beproduced by a conventional device without any modification. Roughlydivided, there are two production methods: method (a) comprising thesteps of melt-kneading a polystyrene resin having a content of organicvolatile matters of less than 100 ppm with or without a cell regulatingagent, injecting a volatile blowing agent into the kneaded mixture undera high pressure to obtain a fluid gel, cooling the fluid gel to atemperature suitable to produce the foam, and extruding the fluid gelinto a low-pressure area to evaporate the volatile blowing agent; andmethod (b) comprising the steps of kneading a polystyrene resin togetherwith water in an extruder, sucking out organic volatile matters througha vent as an azeotrope with water, melt-kneading the kneaded mixturewith or without a cell regulating agent, injecting a volatile blowingagent under a high pressure to obtain a fluid gel, cooling the fluid gelto a temperature suitable to produce the foam, and extruding the fluidgel into a low-pressure area to evaporate the volatile blowing agent.The method (b) can reduce the content of organic volatile matters in theresulting open cell foam irrespective the content of organic volatilematters in the polystyrene resin. In the method (b), a pellet free fromorganic volatile matters may be formed after the steps of kneading apolystyrene resin together with water in an extruder and sucking outorganic volatile matters through a vent as an azeotrope with water andthe open cell foam may be produced from the pellet, although an opencell foam may be produced directly after the above steps.

In the above production methods, control of the temperature of the fluidgel in extrusion foaming is critical. The resin temperature at the timeof extrusion is preferably controlled at 120–170° C. although it may beslightly varied by the physical properties of the polystyrene resin suchas Vicat softening point, melt flow rate and molecular weight.

The temperature at which the polystyrene resin is melted is preferablycontrolled at 180–250° C., although it may slightly varied by thephysical properties of the resin.

A foam obtained as above has skin layers having a foaming degree whichis relatively lower than that of the inner part thereof. The inner partof the foam has an open cell content of 90% or higher, and the skinlayers, which has a closed cell content which is relatively higher thanthat of the inner part, has an open cell content of 0–90%, preferably0–50%. The polystyrene resin foam with skin layers has a density of0.03–0.4 g/cm³, preferably 0.03–0.2 g/cm³, more preferably 0.03–0.1g/cm³, and an open cell content of 50–100%, preferably 60–100%. When thefirst 5.0 mm in thickness (depth) is removed from the surfaces of thefoam, the resulting foam (foam with no skin layer) has an open cellcontent of at least 90%, preferably 95%, more preferably 100%. In thepresent invention, various types of polystyrene foams, whether with ahigh or low foaming rate, whether thick or thin, can be producedaccording to their use by forming such skin layers on the surfaces ofthe foam as in the case with conventional polystyrene resin foams.

Foams having an open cell content of lower than 90% among ones producedby the above method can be processed into an open cell foam of thepresent invention having an open cell content of 90% or higher byremoving the skin layers formed thereon (skin layer formed in theproduction process). The removal of the skin layers can be suitably madewith a belt sander, planer, or the like without forming a film coveringan entire cut surface by fusion of the surface. The thickness of theskin layers of the foam produced by the above method is usually within 5mm, so that removal of a slice thicker than 5 mm results in increase inproduction costs.

As other methods for obtaining an open cell foam having an open cellcontent of at least 90%, there are a method in which a pressure isapplied to an open cell foam having an open cell content of about 70% orlower with a plate or a roll to break closed cells, a method in which aplurality of needles are inserted into such a foam to break closed cellswithout applying a pressure, and a method in which a pressure is appliedto a foam and then a plurality of needles are inserted into the foam.

The open cell foam for use in the present invention has a 5% compressivestrength at 23° C. of at least 0.05 MPa, preferably at least 0.1 MPa.The upper limit thereof is not specifically restricted but is generally2 MPa. When a foam having a 5% compressive strength of lower than 0.05MPa is put in a bag of a gas barrier material such as a foil of a metalsuch as aluminum; a laminated film with an intermediate layer of a gasbarrier material such as polyvinylidene chloride or polyvinyl alcohol;or a low gas permeable plastic film comprising a plastic film ofpolyester or polyethylene on which a metal film is deposited orlaminated and the bag is evacuated to establish a negative pressuretherein, the foam cannot withstand the negative pressure and cell wallsare buckled, resulting in a vacuum heat insulating material with lowheat insulating properties and low thickness accuracy. In producing thevacuum heat insulating material, a getter agent for absorbing watervapor and/or gas is preferably put in the bag of a gas barrier materialtogether with the foam as a core so that the heat insulating propertiesof the vacuum heat insulating material may be maintained for a longperiod of time.

The pressure inside the bag of a gas barrier material of the vacuum heatinsulating material is preferably controlled to 250 Pa or lower inabsolute pressure for reasons of productivity and heat insulatingproperties.

The compressive strength of the foam is measured according to JIS A 9511(1989) as follows. A sample with a size of 50 mm long×50 mmwide×thickness of the foam is compressed in an atmosphere of 23° C. at atest (compression) rate of 10% of the initial thickness of thesample/min. The compressive strength is a value obtained by dividing themaximum load applied to the sample when the sample is compressed in thethickness direction by 5% by the pressure receiving area. The 5%compressive strength of the open cell foam of the present invention ismainly controlled by forming cells of a shape close to a sphere althoughit depends on the physical properties of the polystyrene resin, densityof the open cell foam, shape of the cells, and, in the case of an opencell foam with skin layers which has been compressed, the compressiondegree. More specifically, the 5% compressive strength of the open cellfoam of the present invention is mainly controlled by adjusting theratio of the cell diameter T in the thickness direction to the celldiameter W in the width direction (T/W) and the ratio of the celldiameter T in the thickness direction to the cell diameter L in thelength direction (T/L) in the range of about 0.1–1.5. In view of balancebetween compressive strength and thermal conductivity, the ratios arepreferably in the range of 0.3–1.2.

The open cell foam preferably has a 5% compressive strength at 70° C. ofat least 0.05 MPa so that the core material for a vacuum heat insulatingmaterial can have sufficient heat resistance not to be deformed by heatin a vacuum state. The 5% compressive strength at 70° C. is preferably0.1 MPa or higher for reasons of higher thermal resistance. The upperlimit of the 5% compressive strength at 70° C. is not specificallyrestricted but is generally 1 MPa.

The 5% compressive strength at 70° C. is obtained in the same mannerdescribed as above except that the measurement is performed at 70° C.after a sample has been maintained in an oven at 70° C. for 24 hours.

In the open cell foam of the present invention, the skin layers may beremoved if desired. In this case, the cross-sections of the cells areexposed in the faces of the foam because of the removal of the skinlayers. The cross-sections of the cells may be exposed in one or both ofthe primary faces, or all the faces including the side faces of thefoam. Preferably, the cross-sections of the cells are exposed in all thefaces to bring the open cell content of the foam close to 100%.

A foam obtained by applying a pressure with a plate or a roll withoutremoving the skin layers is preferred because it generates no materialloss and can be produced with high efficiency. A foam with cellscommunicated with each other not by applying a pressure but by insertingneedles exhibits high compressive strength. Especially, an open cellfoam produced without applying a pressure and having an open cellcontent of 90% or higher exhibits excellent compressive strength.

In the vacuum heat insulating material of the present invention, thethermal conductivity 30 days after packing is preferably not greaterthan that immediately after packing for reasons of stable thermalconductivity over a long period of time.

The polystyrene resin open cell foam of the present invention, which isa foam having an open cell structure with an open cell content of atleast 90%, is excellent in heat insulating properties and compressivestrength and thus suitable as a core material for a vacuum heatinsulating material.

The vacuum heat insulating material of the present invention, which isproduced by packing the open cell foam as a core material with a gasbarrier film and evacuating the inside of the package, has a high heatinsulating effect. The heat insulating material, which is used as a heatinsulating panel or the like, is light in weight and easy to handle andcauses less environmental problems in the production process as comparedwith conventional panels made of an inorganic powder or glass fibers.Also, the vacuum heat insulating material of the present invention hasno need for pretreatment such as predrying before vacuum packing and hasgood vacuum maintainability as compared with open cell urethane foams.Also, the vacuum heat insulating material has good recyclability.

Since the open cell foam of the present invention exhibits high heatinsulating properties when used as a core material for a vacuum heatinsulating material and since the open cell foam has a content oforganic volatile matters of less than 100 ppm, there are few factorsthat reduce the vacuum degree after vacuum packing. Thus, the open cellfoam has good vacuum maintainability. In addition, when the open cellfoam is produced through an azeotropic process, there is no need toidentify the content of organic volatile matters of the material. Thus,the open cell foam can be produced with conventional devices and doesnot require very strict production conditions.

The method for producing the foam is characterized by using a blowingagent mainly composed of at least one blowing agent selected frompropane, methyl chloride, ethyl chloride, dimethyl ether, methyl ethylether, and diethyl ether. Thus, an open cell foam having a high opencell content can be easily obtained with high production efficiency.

The vacuum heat insulating material of the present invention isadvantageously used as a heat insulating material for a refrigerator,freezer, insulated truck, refrigerator vessel, freezing vessel,insulated container, insulated box, vender machine, floor heatingsystem, underfloor material, ceiling, and wall.

EXAMPLES

The present invention will be described in detail based on the examples.In the examples and comparative examples, foams were prepared by eitherthe following methods A or B.

Method A

A method for producing a foam using a device having the followingstructure is designated as method A.

A commercially available polystyrene resin is charged in a two-rollextruder having a screw diameter of 47 mm and melted by heating to 220°C. Then, water is pressure-injected into the extruder, and thepolystyrene resin is kneaded. After removing water and organic volatilematters by opening a first vent and sucking with a vacuum pump connectedto a second vent, the kneaded resin is extruded in the form of a strandthrough a lip. The extruded strand is palletized, thereby obtaining apolystyrene resin free from volatile organic matters (pretreatedpolystyrene resin). As a foam production device, an extruder comprisingan extruder having a screw diameter of 65 mm, an extruder having a screwdiameter of 90 mm and an extruder having a screw diameter of 150 mmconnected is used. The extruder having a screw diameter of 150 mm has anend to which a flat die having a resin discharge port (lip) with a widthof 115 mm and a gap of 2 mm is attached. A fluororesin forming implementhaving an entrance which is larger in dimension than the resin dischargeport and an exit with a height of 25 mm and a width of 260 mm. In theforming implement, the height and the width increases gently from apoint in the vicinity of the entrance toward the exit and then becomesconstant.

The pretreated polystyrene resin is supplied to an extruder having ascrew diameter of 65 mm and melted by heating to 220° C. A blowing agentis pressure-injected into the molten resin, which is then kneaded. Thekneaded mixture is further kneaded in the extruder having a screwdiameter of 90 mm and the extruder having a screw diameter of 150 mmsequentially and then extruded into ambient air through the lip and theforming implement, thereby obtaining a molded foam. The thus obtainedfoam has a skin. An open cell foam having a high open cell content canbe obtained by removing skin layers from the top and bottom faces of thefoam.

Method B

A commercially available polystyrene resin is used as it is without anypretreatment such as evacuation. Except that, an open cell foam isobtained in the same manner as in method A.

Example 1

As a raw material resin, a polystyrene resin (impact resistantpolystyrene made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene HI (grade name: H9407), Vicat softening point: 89° C., totalcontent of organic volatile matters: 350 ppm) was used. Talc as a cellregulating agent and methyl chloride as a blowing agent were added tothe raw material resin in amounts of 10 parts by weight and 3.7 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method A. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 128° C. Immediately after the production,the plate-shaped molded foam was moved to a room with a temperature of23° C. and a relative humidity of 50% and aged therein for 20 days.Then, the molded foam was measured for density, open cell content andtotal content of organic volatile matters. The results are summarized inTable 2. The aged plate-shaped molded foam was cut into 200 mm by 200mm, and 3 mm thick slices were cut off of the top and bottom facesthereof (the 200 mm by 200 mm faces) to obtain an open cell foam withoutskin layer (which will be hereinafter referred to as skin-free foam)having a thickness of 12 mm. The physical properties of the thusobtained skin-free foam are shown in Table 2.

The skin-free foam was vacuum-packed (vacuum absolute pressure in thevacuum packing machine: 10 Pa, a getter agent (absorbing agent whichabsorbs air and water vapor)) with a gas barrier laminate film(PET/aluminum foil/low-density polyethylene=12 μm/9 μm/50 μm) to obtaina vacuum heat insulating panel. The thermal conductivity of the vacuumheat insulating panel was measured with a thermal conductivity testermanufactured by EKO INSTRUMENTS TRADING CO., LTD., according to JIS A1412 (1994). The result is shown in Table 1. The heat conductivity 30days after the production is also shown in Table 1. The result in Table1 indicates that the skin-free foam is suitable as a core material for avacuum heat insulating material.

The getter agent does not substantially absorb organic volatile mattersbut mainly absorbs water vapor and inorganic gases.

The heat conductivity of a heat insulating panel obtained by vacuumpacking another plate-shaped molded foam with a gas barrier laminatefilm without removing the skin layers was measured. The result wasalmost the same as that of the insulating panel produced using theskin-free foam of Example 1.

Example 2

A molded foam was produced in the same manner as in Example 1 exceptthat the amount of the cell regulating agent, talc was changed to 5parts by weight. The foaming temperature (temperature of the resinextruded from the extruder with a screw diameter of 150 mm) was set to128° C. Then, a plate-shaped molded foam, a skin-free foam having athickness of 10 mm, and a vacuum heat insulating panel were obtained inthe same manner as in Example 1. Their physical properties aresummarized in Tables 1 and 2. The result in Table 1 indicates that theskin-free foam is suitable as a core material for a vacuum heatinsulating material.

The heat conductivity of a heat insulating panel obtained by vacuumpacking another plate-shaped molded foam with the same gas barrierlaminate film as used in Example 1 without removing the skin layers wasmeasured. The result was almost the same as that of the insulating panelproduced using the skin-free foam of Example 2.

Example 3

As a raw material resin, a polystyrene resin (impact resistantpolystyrene made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene HI (grade name: 433), Vicat softening point: 85° C., totalcontent of organic volatile matters: 250 ppm) was used. Talc as a cellregulating agent and methyl chloride as a blowing agent were added tothe raw material resin in amounts of 5 parts by weight and 3.7 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method A. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 128° C. Then, a plate-shaped molded foam,a skin-free foam having a thickness of 12 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 2. The result inTable 1 indicates that the skin-free foam is suitable as a core materialfor a vacuum heat insulating material.

Example 4

As a raw material resin, a polystyrene type resin (styrene-methacrylicacid copolymer made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene Heat-resistant Type (grade name: G9001), MFR: 1.2 g/10 min,Vicat softening point: 117° C., total content of organic volatilematters: 300 ppm) was used. Talc as a cell regulating agent and methylchloride as a blowing agent were added to the raw material resin inamounts of 10 parts by weight and 3.7 parts by weight, respectively, per100 parts of the raw material resin, and a molded foam was prepared bythe method A. The foaming temperature (temperature of the resin extrudedfrom the extruder with a screw diameter of 150 mm) was set to 151° C.Then, a plate-shaped molded foam, a skin-free foam having a thickness of12 mm, and a vacuum heat insulating panel were obtained in the samemanner as in Example 1. Their physical properties are summarized inTables 1 and 2. The result in Table 1 indicates that the skin-free foamis suitable as a core material for a vacuum heat insulating material.The skin-free foam had a compressive strength at 70° C. of 0.15 MPa.

Example 5

As a raw material resin, a polystyrene type resin (heat-resistant PSPpolystyrene type resin made by Toyo Styrene Co., Ltd., a compound of astyrene-methacrylic acid copolymer comprising 92.2% by weight of styreneunit component and 7.8% by weight of methacrylic acid unit component anda methyl methacrylate-butadiene-styrene rubber (MBS resin) (MBS resincontent: 4% by weight), trade name: Toyo Styrol (grade name: TF-2-311),MFR: 1.0 g/10 min, Vicat softening point: 116° C., total content oforganic volatile matters: 300 ppm) was used. Talc as a cell regulatingagent and methyl chloride as a blowing agent were added to the rawmaterial resin in amounts of 10 parts by weight and 3.7 parts by weight,respectively, per 100 parts of the raw material resin, and a molded foamwas prepared by the method A. The foaming temperature (temperature ofthe resin extruded from the extruder with a screw diameter of 150 mm)was set to 160° C. Then, a plate-shaped molded foam, a skin-free foamhaving a thickness of 12 mm, and a vacuum heat insulating panel wereobtained in the same manner as in Example 1. Their physical propertiesare summarized in Tables 1 and 2. The result in Table 2 indicates thatthe skin-free foam is suitable as a core material for a vacuum heatinsulating material. The skin-free foam had a compressive strength at70° C. of 0.18 MPa.

Example 6

As a raw material resin, a polystyrene type resin (styrene-maleicanhydride copolymer resin made by Nova Chemicals Japan, trade name:Dylark (grade name: 232), MFR: 2.0 g/10 min, Vicat softening point: 112°C., total content of organic volatile matters: 1000 ppm) was used. Talcas a cell regulating agent and methyl chloride as a blowing agent wereadded to the raw material resin in amounts of 10 parts by weight and 3.7parts by weight, respectively, per 100 parts of the raw material resin,and a molded foam was prepared by the method A. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 151° C. Then, a plate-shaped molded foam,a skin-free foam having a thickness of 11 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 2. The result inTable 1 indicates that the skin-free foam is suitable as a core materialfor a vacuum heat insulating material. The skin-free foam had acompressive strength at 70° C. of 0.20 MPa.

The heat conductivity of a heat insulating panel obtained by vacuumpacking another plate-shaped molded foam with the same gas barrierlaminate film as used in Example 1 without removing the skin layers wasmeasured. The result was almost the same as that of the insulating panelproduced using the skin-free foam of Example 6.

Example 7

A polystyrene resin (polystyrene resin for general use made by A & MStyrene Co., Ltd., trade name: A & M polystyrene GP (grade name: 679),Vicat softening point: 102° C., total content of organic volatilematters: 1200 ppm) was used as a raw material resin. Talc as a cellregulating agent and dimethyl ether as a blowing agent were added to theraw material resin in amounts of 5 parts by weight and 3.4 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method A. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 134° C. Then, a plate-shaped molded foam,a skin-free foam having a thickness of 12 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 3. The result inTable 1 indicates that the skin-free foam is suitable as a core materialfor a vacuum heat insulating material.

Example 8

As a raw material resin, a polystyrene resin (impact resistantpolystyrene made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene HI (grade name: H9407), Vicat softening point: 89° C., totalcontent of organic volatile matters: 350 ppm) was used. Talc as a cellregulating agent and dimethyl ether as a blowing agent were added to theraw material resin in amounts of 5 parts by weight and 3.4 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method A. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 128° C. . Then, a plate-shaped moldedfoam, a skin-free foam having a thickness of 14 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 3. The result inTable 1 indicates that the skin-free foam is suitable as a core materialfor a vacuum heat insulating material.

The heat conductivity of a heat insulating panel obtained by vacuumpacking another plate-shaped molded foam with the same gas barrierlaminate film as used in Example 1 without removing the skin layers wasmeasured. The result was almost the same as that of the insulating panelproduced using the skin-free foam of Example 8.

Example 9

As a raw material resin, a polystyrene type resin (heat-resistant PSPpolystyrene type resin made by Toyo Styrene Co., Ltd., a compound of astyrene-methacrylic acid copolymer comprising 92.2% by weight of styreneunit component and 7.8% by weight of methacrylic acid unit component anda methyl methacrylate-butadiene-styrene rubber (MBS resin) (MBS resincontent: 4% by weight), trade name: Toyo Styrol (grade name: TF-2-311),MFR: 1.0 g/10 min, Vicat softening point: 116° C., total content oforganic volatile matters: 300 ppm) was used. Talc as a cell regulatingagent and dimethyl ether as a blowing agent were added to the rawmaterial resin in amounts of 10 parts by weight and 3.4 parts by weight,respectively, per 100 parts of the raw material resin, and a molded foamwas prepared by the method A. The foaming temperature (temperature ofthe resin extruded from the extruder with a screw diameter of 150 mm)was set to 160° C. Then, a plate-shaped molded foam, a skin-free foamhaving a thickness of 14 mm, and a vacuum heat insulating panel wereobtained in the same manner as in Example 1. Their physical propertiesare summarized in Tables 1 and 3. The result in Table 1 indicates thatthe skin-free foam is suitable as a core material for a vacuum heatinsulating material. The skin-free foam had a compressive strength at70° C. of 0.16 MPa.

Example 10

A polystyrene resin (polystyrene resin for general use made by A & MStyrene Co., Ltd., trade name: A & M polystyrene GP (grade name: 679),Vicat softening point: 102° C., total content of organic volatilematters: 1200 ppm) was used as a raw material resin. Talc as a cellregulating agent and propane as a blowing agent were added to the rawmaterial resin in amounts of 5 parts by weight and 3.2 parts by weight,respectively, per 100 parts of the material resin, and a molded foam wasprepared by the method A. The foaming temperature (temperature of theresin extruded from the extruder with a screw diameter of 150 mm) wasset to 134° C. Then, a plate-shaped molded foam, a skin-free foam havinga thickness of 12 mm, and a vacuum heat insulating panel were obtainedin the same manner as in Example 1. Their physical properties aresummarized in Tables 1 and 3. The result in Table 1 indicates that theskin-free foam is suitable as a core material for a vacuum heatinsulating material.

The content of organic volatile matters of the skin-free foams obtainedin Examples 1–10 was below the detection limit.

Comparative Example 1

As a raw material resin, a polystyrene resin (polystyrene resin forgeneral use made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene GP (grade name: 679), Vicat softening point: 102° C., totalcontent of organic volatile matters: 1200 ppm) was used. Talc as a cellregulating agent and methyl chloride as a blowing agent were added tothe raw material resin in amounts of 10 parts by weight and 3.7 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method B. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 134° C. Then, a plate-shaped molded foam,a skin-free foam having a thickness of 10 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 2.

The skin-free foam, which was prepared adding talc at a rate of 10 partsby weight, had a small cell diameter and exhibited quite a low thermalconductivity immediately after the vacuum packing. However, the foam hadas quite a high total content of organic volatile matters as 1000 ppm,so that the vacuum degree was lowered and the heat insulating propertieswere largely decreased as the days went on.

Comparative Example 2

A molded foam was produced in the same manner as in Comparative Example1 except that the amount of the cell regulating agent, talc was changedto 5 parts by weight. The foaming temperature (temperature of the resinextruded from the extruder with a screw diameter of 150 mm) was set to134° C. Then, a plate-shaped molded foam, a skin-free foam having athickness of 11 mm, and a vacuum heat insulating panel were obtained inthe same manner as in Example 1. Their physical properties aresummarized in Tables 1 and 2.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. However, the foam had as quite ahigh total content of organic volatile matters as 1000 ppm as in thecase with Comparative Example 1, so that the vacuum degree was loweredand the heat insulating properties were largely decreased as the dayswent on.

Comparative Example 3

As a raw material resin, a polystyrene resin (H9407, the same one asused in Example 1) was used. Talc as a cell regulating agent and methylchloride as a blowing agent were added to the raw material resin inamounts of 10 parts by weight and 3.7 parts by weight, respectively, per100 parts of the raw material resin, and a molded foam was prepared bythe method B. The foaming temperature (temperature of the resin extrudedfrom the extruder with a screw diameter of 150 mm) was set to 128° C.Then, a plate-shaped molded foam, a skin-free foam having a thickness of10 mm, and a vacuum heat insulating panel were obtained in the samemanner as in Example 1. Their physical properties are summarized inTables 1 and 2.

The skin-free foam, which was prepared adding talc at a rate of 10 partsby weight, had a small cell diameter and exhibited quite a low thermalconductivity immediately after the vacuum packing. However, the foamcontained 300 ppm of organic volatile matters in total, so that thevacuum degree was lowered and the heat insulating properties graduallygot worse as the days went on.

Comparative Example 4

A molded foam was produced in the same manner as in Comparative Example3 except that the amount of the cell regulating agent, talc was changedto 5 parts by weight. The foaming temperature (temperature of the resinextruded from the extruder with a screw diameter of 150 mm) was set to128° C. Then, a plate-shaped molded foam, a skin-free foam having athickness of 12 mm, and a vacuum heat insulating panel were obtained inthe same manner as in Example 1. Their physical properties aresummarized in Tables 1 and 2.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. However, the foam contained 300ppm of organic volatile matters in total, so that the vacuum degree waslowered and the heat insulating properties gradually got worse as thedays went on.

Comparative Example 5

As a raw material resin, a polystyrene resin (433, the same one as usedin Example 3) was used. Talc as a cell regulating agent and methylchloride as a blowing agent were added to the raw material resin inamounts of 5 parts by weight and 3.7 parts by weight, respectively, per100 parts of the raw material resin, and a molded foam was prepared bythe method B. The foaming temperature (temperature of the resin extrudedfrom the extruder with a screw diameter of 150 mm) was set to 128° C.Then, a plate-shaped molded foam, a skin-free foam having a thickness of12 mm, and a vacuum heat insulating panel were obtained in the samemanner as in Example 1. Their physical properties are summarized inTables 1 and 2.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. Although the total content oforganic volatile matters in the foam was 200 ppm, which was the lowestin Comparative Examples, the vacuum degree was lowered and the heatinsulating properties gradually got worse as the days went on.

Comparative Example 6

As a raw material resin, a polystyrene type resin (heat-resistant PSPpolystyrene type resin made by Toyo Styrene Co., Ltd., a compound of astyrene-methacrylic acid copolymer comprising 92.2% by weight of styreneunit component and 7.8% by weight of methacrylic acid unit component anda methyl methacrylate-butadiene-styrene rubber (MBS resin) (MBS resincontent: 4% by weight), trade name: Toyo Styrol (grade name: TF-2-311),MFR: 1.0 g/10 min, Vicat softening point: 116° C., total content oforganic volatile matters: 300 ppm) was used. Talc as a cell regulatingagent and methyl chloride as a blowing agent were added to the rawmaterial resin in amounts of 10 parts by weight and 3.7 parts by weight,respectively, per 100 parts of the raw material resin, and a molded foamwas prepared by the method B. The foaming temperature (temperature ofthe resin extruded from the extruder with a screw diameter of 150 mm)was set to 160° C. Then, a plate-shaped molded foam, a skin-free foamhaving a thickness of 12 mm, and a vacuum heat insulating panel wereobtained in the same manner as in Example 1. Their physical propertiesare summarized in Tables 1 and 3.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. However, the foam had as quite ahigh total content of organic volatile matters as 300 ppm, so that thevacuum degree was lowered and the heat insulating properties werelargely decreased as the days went on.

Comparative Example 7

As a raw material resin, a polystyrene resin (polystyrene resin forgeneral use made by A & M Styrene Co., Ltd., trade name: A & Mpolystyrene GP (grade name: 679), Vicat softening point: 102° C., totalcontent of organic volatile matters: 1200 ppm) was used. Talc as a cellregulating agent and dimethyl ether as a blowing agent were added to theraw material resin in amounts of 5 parts by weight and 3.4 parts byweight, respectively, per 100 parts of the raw material resin, and amolded foam was prepared by the method B. The foaming temperature(temperature of the resin extruded from the extruder with a screwdiameter of 150 mm) was set to 134° C. Then, a plate-shaped molded foam,a skin-free foam having a thickness of 12 mm, and a vacuum heatinsulating panel were obtained in the same manner as in Example 1. Theirphysical properties are summarized in Tables 1 and 3.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. However, the foam had as quite ahigh total content of organic volatile matters as 1000 ppm, so that thevacuum degree was lowered and the heat insulating properties werelargely decreased as the days went on.

Comparative Example 8

As a raw material resin, a polystyrene type resin (heat-resistant PSPpolystyrene type resin made by Toyo Styrene Co., Ltd., a compound of astyrene-methacrylic acid copolymer comprising 92.2% by weight of styreneunit component and 7.8% by weight of methacrylic acid unit component anda methyl methacrylate-butadiene-styrene rubber (MBS resin) (MBS resincontent: 4% by weight), trade name: Toyo Styrol (grade name: TF-2-311),MFR: 1.2 g/10 min, Vicat softening point: 116° C., total content oforganic volatile matters: 300 ppm) was used. Talc as a cell regulatingagent and dimethyl ether as a blowing agent were added to the rawmaterial resin in amounts of 10 parts by weight and 3.4 parts by weight,respectively, per 100 parts of the raw material resin, and a molded foamwas prepared by the method B in the same manner as in Example 6 exceptthat the temperature in the extruder having a screw diameter of 65 mmwas increased to 230° C. The foaming temperature (temperature of theresin extruded from the extruder with a screw diameter of 150 mm) wasset to 160° C. Then, a plate-shaped molded foam, a skin-free foam havinga thickness of 14 mm, and a vacuum heat insulating panel were obtainedin the same manner as in Example 1. Their physical properties aresummarized in Tables 1 and 3.

The skin-free foam exhibited a relatively low thermal conductivityimmediately after the vacuum packing. However, the foam had as quite ahigh total content of organic volatile matters as 350 ppm, so that thevacuum degree was lowered and the heat insulating properties werelargely decreased as the days went on.

The content of organic volatile matters in each of the plate-shapedmolded foams of Examples 1–10 and Comparative Examples 1–8 shown inTables 2b and 3b is a value measured after one week of aging afterextrusion. The content of organic volatile matters in each of theskin-free foams was measured after removing the skin layers after oneweek of aging after extrusion.

TABLE 1 Pressure Getter X Y Y − X Example * (1) * (2) * (3) * (4) * (5)1 10 Used 0.0100 0.0093 −0.0007 2 10 Used 0.0122 0.0118 −0.0004 3 10Used 0.0121 0.0117 −0.0004 4 10 Used 0.0086 0.0081 −0.0005 5 10 Used0.0081 0.0079 −0.0002 6 10 Used 0.0080 0.0075 −0.0005 7 10 Used 0.01180.0115 −0.0003 8 10 Used 0.0119 0.0115 −0.0004 9 10 Used 0.0082 0.0077−0.0005 10  10 Used 0.0113 0.0109 −0.0004 Comp. 1 10 Used 0.0092 0.01200.0028 Comp. 2 10 Used 0.0121 0.0164 0.0043 Comp. 3 10 Used 0.01000.0122 0.0022 Comp. 4 10 Used 0.0122 0.0158 0.0036 Comp. 5 10 Used0.0121 0.0157 0.0036 Comp. 6 10 Used 0.0079 0.0117 0.0038 Comp. 7 10Used 0.0119 0.0162 0.0043 Comp. 8 10 Used 0.0078 0.0115 0.0037 * (1):Vacuum degree in packing device, Absolute pressure (Pa) * (2): Getteragent * (3): Thermal conductivity immediately after packing (X) (W/mK) *(4): Thermal conductivity 30 days after packing (Y) (W/mK) * (5): (Y) −(X) (W/mK)

TABLE 2a Amount Amount of of volatile volatile Foaming Name mattersMethod matters temperature Example * (1) * (2) * (3) * (4) (° C.) 1H9407 350 A Below 128 detection limit 2 H9407 350 A Below 128 detectionlimit 3 433 250 A Below 128 detection limit 4 G9001 300 A Below 151detection limit 5 TF-2- 300 A Below 160 311 detection limit 6 232 1000 A Below 151 detection limit Comp. 1 679 1200  B Not treated 134 (1200)Comp. 2 679 1200  B Not treated 134 (1200) Comp. 3 H9407 350 B Nottreated 128 (350)  Comp. 4 H9407 350 B Not treated 128 (350)  Comp. 5433 250 B Not treated 128 (250)  * (1): Material grade name * (2): Totalcontent of organic volatile matters in commercially availablepolystyrene resin (ppm) * (3): Production method * (4): Total content oforganic volatile matters after treatment (ppm)

TABLE 2b-1 Plate-shaped molded foam Open cell Total content of Densitycontent organic volatile Example (g/cm³) (%) matters (ppm) 1 0.100 98Below detection limit 2 0.095 94 Below detection limit 3 0.099 89 Belowdetection limit 4 0.069 81 Below detection limit 5 0.081 80 Belowdetection limit 6 0.100 92 Below detection limit Comp. 1 0.093 81 1000Comp. 2 0.067 93 1000 Comp. 3 0.101 99  300 Comp. 4 0.096 95  300 Comp.5 0.099 87  200

TABLE 2b 2 Skin-free foam Cell diameter Total content Compres- Open inof organic sive cell thickness volatile strength Density contentdirection matters at 23° C. Example (g/cm³) (%) (μm) (ppm) (MPa) 1 0.102100 120 Below 0.17 detection limit 2 0.081 100 220 Below 0.16 detectionlimit 3 0.088 100 210 Below 0.18 detection limit 4 0.083 100 100 Below0.36 detection limit 5 0.080 100 100 Below 0.45 detection limit 6 0.106100  90 Below 0.56 detection limit Comp. 1 0.095 100 110 1000 — Comp. 20.066  98 220 1000 — Comp. 3 0.107 100 120  300 — Comp. 4 0.080 100 220 300 — Comp. 5 0.087 100 210  200 —

TABLE 3a Amount Amount of of volatile volatile Foaming Name mattersMethod matters temperature Example * (1) * (2) * (3) * (4) (° C.)  7 6791200 A Below 134 detection limit  8 H9407  350 A Below 128 detectionlimit  9 TF-2-  300 A Below 160 311 detection limit 10 679 1200 A Below134 detection limit Comp. 6 TF-2-  300 B Not treated 160 311 (300) Comp. 7 679 1200 B Not treated 134 (1200) Comp. 8 TF-2-  300 B Nottreated 160 311 (300)  * (1): Material grade name * (2): Total contentof organic volatile matters in commercially available polystyrene resin(ppm) * (3): Production method * (4): Total content of organic volatilematters after treatment (ppm)

TABLE 3b-1 Plate-shaped molded foam Open cell Total content of Densitycontent organic volatile Example (g/cm³) (%) matters (ppm)  7 0.062 77Below detection limit  8 0.091 90 Below detection limit  9 0.066 83Below detection limit 10 0.055 78 Below detection limit Comp. 6 0.085 82 300 Comp. 7 0.062 75 1000 Comp. 8 0.068 85  350

TABLE 3b-2 Skin-free foam Cell diameter Total content Compres- Open inof organic sive cell thickness volatile strength Density contentdirection matters at 23° C. Example (g/cm³) (%) (μm) (ppm) (MPa)  70.068 100 200 Below 0.13 detection limit  8 0.082 100 190 Below 0.17detection limit  9 0.078 100  90 Below 0.46 detection limit 10 0.066 100160 Below 0.14 detection limit Comp. 6 0.081 100 100  300 — Comp. 70.067 100 210 1000 — Comp. 8 0.081 100  90  350 —

The physical properties in the tables 2 and 3 were measured as follows.

Density:

Measurement was performed according to the method of measuring apparentdensity provided in JIS K6767 (1976).

Open Cell Content:

The method has been described before.

Content of Organic Volatile Matters:

The method has been described before.

In the chromatograms obtained in the measurement of organic volatilematters in the foams, no peak was detected except those of cyclopentanolas the internal standard substance and dimethylformamide as a solvent.

Compressive Strength:

The method has been described before.

The thermal conductivity in Examples and Comparative Examples wasmeasured by the following method.

Thermal Conductivity:

Measurement was performed by a plate heat flow meter method (twin-platetype, average temperature: 23° C.) provided in JIS A1412 (1994). Thesample for measurement was a vacuum packed core material (foam fromwhich the skin layers on both sides had been removed) for a vacuum heatinsulating material having a size of 200 mm (length)×200 mm(width)×thickness of the sample.

1. A vacuum heat insulating material comprising: an open cell foam corecomprising a polystyrene resin foam having an open cell content of atleast 90%, a density of 0.03–0.4 g/cm³ and a cell diameter in thedirection of the thickness of said foam of less than 400 μm, saidpolystyrene resin foam having a content of organic volatile matter ofless than 100 ppm, said organic volatile matter having a boiling pointof not higher than 160° C. under ambient pressure; and a barrier filmcovering said open cell foam core in an evacuated state; wherein saidvacuum heat insulating material is produced by covering said open cellfoam core with the gas barrier film, and evacuating the interior of thegas barrier film.
 2. A vacuum heat insulating material as recited inclaim 1, wherein said polystyrene resin foam has a content of saidorganic volatile matter of 50 ppm or less.
 3. A vacuum heat insulatingmaterial as recited in claim 1, wherein said polystyrene resin foam hasa content of said organic volatile matter of less than 20 ppm.
 4. Avacuum heat insulating material as recited in claim 1, wherein saidpolystyrene resin foam has a cell diameter in the direction of thethickness of said foam of 80 to 200 μm.
 5. A vacuum heat insulatingmaterial as recited in claim 1, wherein said polystyrene resin foam hasa thickness of 5.0 to 50.0 mm.
 6. A vacuum heat insulating material asrecited in claim 1, wherein said open cell foam core has a density of0.03 to 0.2 g/cm³ and a 5% compressive strength at 23° C. of at least0.05 MPa.
 7. A method for producing an open cell foam according to claim1 having a content of organic volatile matters of less than 100 ppm,wherein said method comprises melt-kneading a polystyrene resin having acontent of organic volatile matters of less than 100 ppm together with ablowing agent in an extruder, and then extruding the kneaded mixtureinto a lower pressure area to cause said polystyrene resin to foam.
 8. Amethod for producing an open cell foam as claimed in claim 7, whereinsaid blowing agent comprises, as a main component, at least one blowingagent selected from propane, methyl chloride, dimethyl ether, methylethyl ether and diethyl ether.
 9. A method for producing an open cellfoam according to claim 1, wherein said method comprises melt-kneading apolystyrene resin together with water in an extruder, sucking outvolatile matters through a vent as an azeotrope with water, injecting ablowing agent and kneading said polystyrene resin therewith, andextruding 10 the kneaded mixture into a lower pressure area to causesaid polystyrene resin to foam.